Method of changing the thermochromic transition temperature of silver mercuric iodide and copper mercuric iodide



United States Patent ABSTRACT OF THE DISCLOSURE The method of changing the thermochromic transition 3,516,945 Patented June 23, 1970 Photochromics are another class of materials that can be possibly employed in display devices for high ambient illumination situations. These materials change color by absorption of light of a particular wavelength. The color can be erased by heating or by illumination of the colored form of the material by light of longer wavelength than was used to accomplish the initial color change. These materials are usually white or light yellow in color and usualtemperature in material selected from the group consisting of silver mercuric iodide and copper mercuric iodide by the partial ionic substitution on the mercuric iodide side of the material by an element of a lower period in the same group, said element being cadmium when the substitution is made of the mercury element and said element being selected from the halide group consisting'of chlorine and bromine when the substitution is made of the iodide element.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to thermochromic ma terials andmore particularly to the regulation of the transitions temperature of such materials.

Description of-the prior art Materials which undergo a color change when theyare heated above a certain temperature (usually called the transition temperature) and revert back to their original color when cooled-belowtlre'transition temperature are called thermochromics. The phenomenon of thermochromism has been known for some time, but the application of these thermochromic materials to a practical display device has proved exceedingly difficult. 7.

Display devices utilizing electroluminescent and cathodoluminescent materials, the so-called phosphors, are well documented, as is evident by the existence of such products as alpha-numeric electroluminescent panels and cathode ray tubes. In these display devices the material is emitting light (luminescence) as a result of the application of an electric field in the case of electroluminescent materials and bombardment by an electron beam in the 1y require ultraviolet illumination to change color. If the photochromic material is used alone as the target material, two light sources are necessary. In order to write on the photochromic scanning of the ultraviolet beam, either a mechanical or electro-optic method is required. A scanning method is also required for the erasure beam. Usually only mechanical scanners are practicable. Heating the target material may replace the erasure light beam SUMMARY OF THE INVENTION Generally speaking, the present invention contemplates the alteration and control of the transition temperature of silver mercuric iodide, or copper mercuric iodide by (a) the partial anionic substitution of the iodine with chlorine or bromine, (b) the partial cationic substitution of the mercury with cadmium, or (c) a combination of both of the aforementioned anonic and cationic substitutions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The starting materials and their thermochromic properties are listed in Table I.

TABLE I.-THERMOCHROMIC INORGANIC TERNARY Copper mercuric iodide CllzHgLi -IODIDES AND THEIR THERMOOHROMIC PROPERTIES Chemical Transition formula temp.,C. Color change Silver mercuric iodide. AgzHgIl Compound name 50.5 Yellow-orange-red.

69 Red-black.

' perature of between approximately 250 C. and 400 C.

case of cathodoluminescent materials. Since these displays depend on the emission of light, the brightness is one of the prime factors in evaluating them. Oneof the main limitations of suchjdeyices is the"bri'g'l 1tness levels that have been achieved. They-are not-sufficiently bright to be viewed under high ambient illumination, Attempts to increase the brightness by increasing the electric field and its frequency liave resultediri a lowering ofthe lifetime of material. Thus, these devices have found utility only in situations where the ambient illumination is not too high- Thermochromic materials,--however, do not have this limitation because the color change in these materials is viewed by reflected light. Thus, the display can be read under high ambient-illumination. Devices employing for some sixty odd hours, and forming a powder from the resultantagglomerate. j. For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative examples. are given to show how the products desired are obtained. I EXAMPLE 1 I 2 aa4 loJs HgI .0301 gram of AgBr and 0.4320 gram of AgI' were weighed}- These ingredients were mixed in a Wig-L-Bug (Crescent Dental Mfg. Co.) for several minutes. The: mixture was then placed: in Pyrex tubing which was thene'vacuated and-sealed 'oif. The

' seale d'tube 'was' placed in 'a' furnace at 350 C. for 64 thermochromics, however; cannot-be-read when the light ambient is too low. 'Ijhus, devices employing thermochromics could be used. in situations where itis not'practical to eliminate the ambient illumination, such as in the cockpit of an airplane. I

p 0.9089 gram of H'gl 0.8218 gram of AgI, and 0.0717

the Pyrex tube and ground in the gram of AgCl were weighed. These ingredients were mixed in a Wig-L-Bug for one minute. The mixture was placed in a Pyrex tube which was evacuated and sealed. They were placed in a furnace at 250 C. The furnace temperature was raised to 350 C. The sample remained at this temperature for about 64 hours. The sample was then cooled to room temperature, removed from the Pyrex tubing, and ground to a powder in a Wig-L-Bug.

EXAMPLE 3 z avs azs 0.9089 gram of HgI 0.6666 gram of CuI, and 0.0495 gram of CuCl were weighed. These compounds were mixed in a Wig-L-Bug for one minute. The mixture was placed in a Pyrex tube which was evacuated and sealed. The same firing schedule described for Ag HgI Cl was followed.

EXAMPLE 4 z oso o. 1o 3.e4 0.16

0.4761 gram of AgI, 0.0331 gram of AgBr, 0.4507 gram of HgI and 0.0407 gram of CdI; was weighed out. These compounds were placed in a Wig-L-Bug for 5 minutes. The mixture was then placed in a Pyrex tube which was evacuated and sealed. The same firing schedule was used for this formulation as was used for AggHgmgocdmmL The sample had melted. After 5 minutes of milling in a Wi-L-Bug a yellow powder resulted.

0.9217 gram of CuI, 0.9900 gram of Hgl and 0.0887 gram of Cdl were weighed out. These compounds were mixed for 5 minutes in a Wig-L-Bug. The mixture was placed in a Pyrex tube which was evacuated and sealed. The sample was placed in a furnace at 400 C. The same procedure described previously for the other partial cationic substitution formulations was then followed.

EXAMPLE 6 z sxzs 0.25

0.9089 gram of HgI 0.6666 gram of CuI, and 0.0717 gram of CuBr were weighed. These compounds were mixed in a Wig-L-Bug for one minute. The mixture Was placed in a Pyrex tube which was evacuated and sealed. The same firing schedule described for Ag HgI Cl was also used for this formulation.

EXAMPLE 7 This formulation was synthesized by weighing out 0.5128 gram of AgI, 0.4472 gram of HgI and 0.0400 CdI These ingredients were placed in a Wig-L-Bug for 5 minutes. The mixture was put into a Pyrex tube which was evacuated and sealed. The sample was placed in a Lindbergh furnace at 405 5C. for five days and 17 hours. It was removed from the furnace after this time and quenched to room temperature in 30 minutes. After grinding in the Wig-L-Bug an ochre yellow powder resulted.

The materials obtained in the foregoing examples as well as other materials produced in a similar manner were then tested for thermochromic properties and the results were then tabulated for transition temperature and cold and hot wavelengths.

The A and A were obtained from the reflectance spectra of these formulations measured at 25 C. and 90 10 C., respectively. The A corresponds to the wavelength of the cold form and the M, corresponds to the wavelength of the hot form. These wavelengths correspond to the reflected intensity which lies halfway between its long wavelength maximum value and its short wavelength minimum value at a particular temperature.

When subjected to anionic substitution, all of the silver containing formulations underwent a thermochromic change from yellow to orange and the copper containing formulations changed color from red to black The transition temperatures of these systems were measured by the melting point capillary method. Partial anionic substitution by either Br or Cl caused a lowering in the transition temperature from 50.5 C. to 42 2 C. for the Ag HgL; and from 69 C. to 50 C. for Cu HgI There was no linear relation between the amount of bromide ion substituted and the transition temperature. Optimum thermochromic properties for the bromine substituted formulations occurred at 4 percent bromide ion. As the bromide ion content was increased above 4 percent the thermochromic color changed and became more gradual with temperature.

When subjected to cationic substitution, all of the silver-containing fonmulations undergo a thermochromic change from yellow to orange, which is similar to the change observed in Ag HgI The copper-containing formulations all went from red to black as is the case with Cu HgI In general, there is a rise in transition temperature with :increasing cadmium content. The transition temperatures for these formulations were measured by the melting point capillary method and the Dennis melting point method which makes use of a temperature gradient stage. The transition temperature range was much broader in the case of the parent compounds or the bromine substituted ternary iodides.

The tabulated results are shown in Tables 2 and 3.

TABLE 2.ANIONIC SUBSTITUTIONAL FORMULATIONS TABLE 3.OATIONIC SUBSTITUTIONAL FORMULATION S [Example 7] Agi gr-xCdJt Transition Temperature Range 0.)

Temperature Melting gradient point x stage capillary QUINN OUIOCHO PPPPPPPPPPPPPPQ Transition Temperature ange 0.)

Temperature Melting gradient point 1: stage capillary [Example 5] CuzHgi-xCdxh Transition Temperature It is to be observed therefore that the present invention provides for a method of changing the thenmo chromic transition temperature in material selected from the group consisting of silver mercuric iodide and copper mercuric iodide by the partial ionic substitution on the mercuric iodide side of the material by an element of a lower period in the same group (as appearing .in the Periodic Table of Elements, e.g., Websters Seventh New Collegiate Dictionary, G. & C. Merriam Company, Springfield, Mass. 1965 ed., p. 628) said element being cadmium when the substitution is made of the mercury element and said element being selected from the halide group con sisting of chlorine and bromine when substitution is made of the iodine element. The substitution is made by taking mercuric iodide on the one hand and silver iodide or copper iodide on the other hand, together with the corresponding silver copper halide for the anionic substitution and cadmium iodide for the cationic substitution, first mixing these ingredients and then heating them in a vacuum at temperatures of between about 250 C. and about 400 C. for a time period of the order of sixty hours, and then forming a powder of the resultant agglomerate.

While the present invention has been described in preferred embodiments, it will be obvious to those skilled in the art that various modifications can be made therein within the scope of the invention, and it is intended that the appended claims cover all such modifications.

What is claimed is:

1. The method of chang'mg the thermochrornic transition temperature in material selected from the group consisting of silver mercuric iodide and copper mercuric iodide by the partial ionic substitution on the mercuric iodide side of the material by an element of a lower period in the same group,'said element being cadmium when the substitution is made of the mercury element and said element being selected from the halide group consisting of chlorine and bromine when the substitution is made of the iodine element.

2. The method claimed in claim 1, wherein said substitution is cationic, and an element selected from the chlorine and bromine group is partially substituted for the iodine.

3. The method as claimed in claim 1, wherein the substitution is anionic, and cadmium is partially substituted for the mercury.

4. The method claimed in claim 1, wherein mercuric iodide on the one hand and material selected from the group consisting of silver iodide and copper iodide on the other hand, together with the corresponding silver copper halide for the anionic substitution and cadmium iodide for the cationic substituiton are first mixed and then heated in a vacuum at temperatures between about 250 C. and about 400 C. for some time of the order of sixty hours, and then forming a powder of the resultant agglomerate.

5. The methodof claim 1, wherein chlorine is partially substituted for the iodine.

6. The method of claim 1, wherein bromine is partially substituted for the iodine.

7. The method of claim 1, wherein cadmium is partially substituted for the mercury.

References Cited UNITED STATES PATENTS 3,352,794 11/1967 Abdo 252-408 2,945,305 7/ 19 Strickler.

PHILIP DIER, Primary Examiner M. E. McCAMISH, Assistant Examiner US. Cl. X.R. 7335 6 

